Praise be to Allah the Lord of the Worlds.
According to Islam, a woman isn`t required to look after her husband`s parents, unless she volunteered to do so and Allah the Almighty will reward her for that. Abu Hurairah (May Allah be pleased with him) reported: The Prophet (PBUH) said, "I saw a man going about in Jannah (and enjoying himself) as a reward for cutting from the middle of the road, a tree which was causing inconvenience to the Muslims." [Muslim].
Since Allah has rewarded that man for cutting a tree that was causing inconvenience to Muslims, He will reward a righteous woman for taking care of her husband`s parents and being dutiful to them.
Accordingly, we advise the above questioner to exercise patience as regards the accusations made by her brothers and sisters in law and try to clarify the truth about every situation. If that didn`t pay off, then keep serving your parents-in-law and Allah will reward you for that. And Allah the Almighty knows best.